The roles which water and exchangeable protons play in stabilizing the physiologically active structures of proteins involved in oxygen transport, storage and utilization will be investigated. In hemeproteins changes in subunit interactions accompanying the substitution of heavy for ordinary water express themselves in part, and most importantly in relation to the active site, through changes in the environment, binding, and hence electronic structure of the heme group. Electron paramagnetic resonance and light absorption will be used to study changes in the iron and porphyrin moieties of hemeproteins which occur when heavy water is substituted for ordinary water in solutions of these proteins, and when chemical and physical agents which affect water and protein structure are introduced.